The present invention relates to the field of electrical initiators and gas generators. More particularly, the present invention relates to electrical initiators used to ignite gas generators for inflating air bags and for electrically initiated gas generators for seat-belt pretensioners in automobiles during collisions.
Air bags and seat belt pretensioners play an important role in reducing death or injuries in collisions. An initiator has a crucial role in activating these safety mechanisms by quickly converting an electrical signal from a collision detection system to rapidly moving, hot particles. These hot particles ignite a solid gas generant which in turn produces the gas necessary to inflate an air bag or activate a seat-belt pretensioner.
Conceptually, an electrical initiator contains a number of components. It has a header and a cup that are attached together to form a cavity. An initiator also has two electrically conductive pins that provide a conduction path from the outside of the header and cup into the cavity. Inside the cavity, the pins are connected together by an electrically resistive device, called a resistor in this discussion.
When the resistor is composed of a piece of metal, the resistor is called a bridgewire.
The resistor is surrounded by a chemical compound called the primer that is very sensitive to temperature. Adjacent to the primer is another chemical compound called the output charge. The output charge and the primer together are referred to as the ordnance. The ordnance is contained by the formed cavity.
The initiator is contained in a device called a gas generator. For simplicity in describing the operation of an initiator in the context of a safety system, the cup of the initiator can be thought of as being surrounded by a solid chemical called the gas generant. When the solid gas generant is ignited, it produces a gas.
The operation of an initiator begins with the arrival of an electrical signal at the conductive pins. The resistor converts the electrical energy in the signal into thermal energy. That thermal energy causes the resistor temperature to rise which starts a pyrotechnic reaction in the primer. The pyrotechnic reaction in the primer causes a pyrotechnic reaction in the output charge. The increased pressure and heat generated by these reactions causes the cup to rupture. The high pressure spreads hot gases and particles outward to ignite the solid gas generant to produce gas. This gas can then be used to inflate an air bag or move a piston to operate a seat belt pretensioner.
A commercially successful initiator used in automotive safety systems must be fast, reliable and consistent. It also must be economical to construct.
An initiator must be reliable and fast because it must reliably ignite when required and never ignite unintentionally. An initiator can spend years unused in a car before it needs to work. It must be fast because the gas generators must inflate an air bag or tighten a seat belt in time to prevent injury to the automobile occupants. It must be fast so that the safety system designers can make sure that all parts of the safety system work at the precisely the proper time to provide the protection to the occupants.
Some initiators requiring high reliability and consistency use a metal header and employ a glass-to-metal seal or a ceramic-to-metal seal between the pins and the header, and have a metal cup welded to the header. In these initiators one or both pins are fed through the metal header via a glass or ceramic insulator which seals the metal pin to the insulator and the insulator to the metal header. If only one pin is insulated from the header, the header itself acts as part of the conductive path to the cavity.
The glass-to-metal seal or ceramic-to-metal seal is a hermetic seal and is strong enough to hold the pin or pins in place during the time that the initiator is operating. These types of seals isolate the resistor, the primer, and the output charge from external moisture and humidity fluctuations. Moisture in the ordnance reduces the initiator's ability to fire promptly and consistently upon receipt of the proper electrical signal.
An initiator must be economical to build. Glass-to-metal, ceramic-to-metal and metal-to-metal welded seals are expensive. They may be the most expensive aspect of constructing an initiator. Unfortunately, initiators using less expensive materials such as nylon are much less reliable. For instance, an initiator may use a plastic header and cup. Sometimes initiator manufacturers attempt to provide an environmental seal between the header and cup by use of crimps or potting material. Although this type of initiator is less expensive, it does not provide a seal suited for the demands of the automotive environment, nor is it able to provide the long term reliability critical for this type of safety application.
Existing initiators using plastic are not effective in isolating the primer and output charge from the environment. A path for the intrusion of moisture may exist between the pins and the plastic header. For example, some initiators are constructed by molding the pins in the header. The header may pull away from the pins when the injected plastic cools, thus leaving a path for moisture.
Plastic headers and cups have very large coefficients of thermal expansion compared to glass-to-metal headers. Expansion and contraction over a long lifetime, e.g. 15 years, in an automotive environment can mechanically stress the resistor. Fractures in the resistor can cause electrical problems that lead to late firing of the initiator or even complete failure.
Some initiators have the resistor attached to the pins with solder. One problem with this approach is that the solder flux can contaminate the primer. Soldering also does not guarantee a reliable connection. Both of these problems can make the initiator unreliable. In addition, soldering requires additional materials, i.e. solder and flux. This makes an initiator using these materials more difficult and expensive to build than one without those materials.
When properly deployed, the initiator will receive an electrical signal from the sensing system. However, the initiator can be inadvertently triggered by static electricity generated while the initiator is being built or installed. This creates a substantial safety hazard to workers and equipment.
The ideal output charge would have several important characteristics. It would maintain its ignition and combustion characteristics in the presence of moisture. It would produce numerous hot particles to ignite the gas generant. It would also be relatively insensitive to ESD. Although far from ideal, many initiators use black powder as an output charge.
Initiators have used a primer composed of normal lead styphnate (NLS) with nitrocellulose as a binder. However, this primer does not have good heat transfer properties and will fail the no-fire requirement unless a large diameter bridgewire is used, or the primer's heat transfer characteristics are modified. A typical no-fire requirement is that the primer must not ignite 99.9% of the time with a 95% confidence level at 200 milliamps applied for 10 seconds at 85.degree. C. However, a larger bridgewire will cause the initiator to have a slower response time, which may lead to failing the response time requirement and the all-fire requirement. A typical all-fire requirement is that the primer must ignite 99.9% of the time with a 95% confidence level at 800 milliamps applied for 2 milliseconds at -35.degree. C.
Because nitrocellulose is less thermally stable than normal lead styphnate and because it does not provide the primer with good heat transfer characteristics, primers using nitrocellulose have poor long term aging characteristics, poor thermal heat sink capability, and lack the required resiliency to survive thermal and mechanical shock readily. The lack of resiliency means that the primer is stiff and brittle, and therefore is incompatible with an ultrasonic welding process.